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Days after a young Jersey City Police officer was killed in the line of duty, News 4 has obtained an internal police memo warning that a gang is recruiting members across New Jersey to kill more officers. Brian Thompson has more. (Published Wednesday, July 16, 2014)

Updated at 7:02 AM PDT on Thursday, Jul 17, 2014

Police in Jersey City are being warned after an internal department memo obtained by NBC 4 New York says a gang there is recruiting gang members across New Jersey to kill cops after a rookie police officer was slain responding to a call of an armed robbery over the weekend.

Melvin Santiago was killed early Sunday while responding to an armed robbery call at a 24-7 Walgreens. Police say Lawrence Campbell ambushed the 23-year-old outside of the store. Police returned fire, killing Campbell.

Jersey City Mourns Fallen Officer

People gathered Monday at the scene where Officer Melvin Santiago was gunned down over the weekend. Brian Thompson reports. (Published Tuesday, July 15, 2014)

According to the memo, "information from a reliable source" within the gang indicates the gang is asking for help from members in Camden and Newark in shooting another police officer.

The gang is targeting officers working with a Department of Transportation traffic unit, the memo says. The advisory warns gang members will "attack and shoot Jersey City EMTs because they know police officers will respond immediately to their backup."

Jersey City Police Officer Killed in Drugstore Shooting

A Jersey City police officer was shot in the head and killed early Sunday by a man who told a witness he was "going to be famous," Mayor Steven Fulop said. Ida Siegal reports (Published Monday, July 14, 2014)

Mayor Steven Fulop said such threats frequently crop up after police shootings and are rarely credible.

Officer Killed in Jersey City Shooting

A Jersey City police officer was shot in the head and killed early Sunday by a man who told a witness he was "going to be famous," Mayor Steven Fulop said. Sheldon Dutes reports. (Published Sunday, July 13, 2014)

"Every urban area, every police department knows that after every single police shooting, threats come in," Fulop said. "Ninety-nine-point-nine percent of the time they're not credible, but we always take them seriously."

Police sent a notice about the potential threat to officers. One of them posted it to his Facebook page. Fulop said that officer would be disciplined for possibly violating department protocols.

Santiago was one of the first officers at the scene after the report of the armed robbery at the Walgreens came in Sunday by Campbell, who allegedly told someone inside the store to watch the news later because he was "going to be famous."

Campbell didn't try to rob the store, but assaulted its armed security guard and snatched his gun, officials have said. He then waited for officers to arrive and shot Santiago with what police believe was the guard's weapon.

Jersey City Mayor Steven Fulop has said Campbell, who had prior drug arrests and was released from jail in January, was one of three suspects wanted by police for a prior homicide. Another man being sought in that case, 23-year-old city resident Daniel Wilson, was captured Sunday night, officials said.

Since Sunday, tensions have risen over a sidewalk memorial set up for Campbell in the crime-plagued neighborhood where he lived. Fulop ordered the memorial taken down Tuesday.

The mayor met Wednesday evening with 50 church leaders to talk about the recent clashes with police and growing community unrest. Outside the building, armed SWAT team members stationed themselves next to the small crowd gathered there. Some neighbors were upset that the public wasn't allowed to take part in the meeting.

"It's just crazy, they have clergy up there that's not even from this community," said Henry Todd.

But Fulop said the clergy offered constructive input.

"Those people are not people coming here just to make noise and stir the pot," he said. "They're there looking for solutions."

Fulop said Santiago will be promoted to the rank of detective and receive the department's Medal of Honor. The promotion and ceremony will happen at his wake Thursday. His funeral is scheduled for Friday.

Gov. Christie ordered flags be flown at half-staff Wednesday in his honor.